In recent years, fluorine-containing polymers are used for various applications, utilizing their characteristics as high polymer materials excellent in heat resistance, solvent resistance, chemical resistance, etc.
As a method of producing a fluorine-containing polymer, a solution polymerization method, a suspension polymerization method or an emulsion polymerization method has been known. As a polymerization medium of the solution polymerization method or the suspension polymerization method, an inert solvent such as a chlorofluorocarbon (CFC) has been commonly used, since a copolymer having a high molecular weight can be obtained or the speed of polymerization is high. As a specific example of CFC, trichlorofluoromethane, dichlorodifluoromethane, trichlorotrifluoroethane or dichlorotetrafluoroethane may be mentioned. Among these, trichlorotrifluoroethane has been mainly used in view of handling.
However, destruction of the ozone layer has been internationally taken up as an environmental problem of global scale, and use of certain CFCs which are considered to be causal substances has been prohibited Therefore, there is substantial demand for developing a substitute for CFC to be used for producing a fluorine-containing polymer.
As a substitute for CFC, a hydrofluorocarbon (HFC) having hydrogen atoms has been proposed, as it has a small ozone destruction coefficient. However, it is known that a substance having C--bonds shows a chain transfer property to a fluoroolefin. Accordingly, it may sometimes be difficult to use it as a polymerization medium for producing a fluoroolefin polymer having a high molecular weight. As another substitute for a polymerization medium, e.g. t-butanol has been known (JPB-52-24073). However, it is necessary to carry out polymerization under high pressure, in order to obtain a copolymer having an adequately high molecular weight.